1. Field of the Invention
This invention relates generally to single-stage, mechanically driven steam turbines, and is concerned in particular with an improved support structure for reliably positioning and maintaining such turbines in accurate alignment with associated driven equipment such as pumps, blowers, fans, compressors and the like.
2. Description of the Prior Art
Single-stage, mechanically driven steam turbines have been in common use as prime movers in industry since the late 19th century. Early turbine designs were influenced by the designs of other contemporary machinery and mechanical devices. For example, early turbines often had supports that were formed as integral foot-like projections from the larger pressure containing vessels or casings. That is, they had integrally-cast legs, culminating in load-bearing, horizontal flat pads at the lower, outboard extremities of the main casing, much like the four "feet" of the then contemporary lathes and bath tubs. The feet were firmly bolted in place, and served effectively as simple yet rugged supports.
The steam used to drive the early turbines was most often generated at fairly low pressures and was usually saturated (unsuperheated), a condition which is considered relatively "cool" by today's standards. Turbine exhaust pressures and corresponding exhaust temperatures were also comparatively mild.
When a turbine is started from cold, stand-still conditions, the pressure casing soon warms to nearly the same temperature as the steam powering the turbine. As the casing warms, it undergoes thermal expansion in all directions. However, in the early turbine designs, thermal expansion was resisted by the support feet, which were bolted in place and thus restrained from freedom of movement. This resulted in the casings being subjected to significant stress and strain. This problem was compounded by the fact that as the casings expanded upwardly from the supporting foot pads, the turbine rotor shafts also underwent upward vertical displacement as well as axial expansion, resulting in alignment problems with respect to the equipment being driven by the turbines.
In the early years of low pressures and temperatures, these turbine thermal expansion dislocations, stresses and strains were largely inconsequential. But as years went by and boiler technology improved, steam pressures and temperatures increased progressively, as did turbine operating speeds. The thermal dislocations and the thermally-induced stresses and strains then became significant. Turbine shaft alignment to the driven equipment was kept within reason when in operation, by initially biasing the turbine below and away from the driven equipment when cold (called cold alignment), so that as turbine components expanded on warm-up, the turbine shaft would thermally move into satisfactory alignment. Later, so-called flexible couplings came into common use, providing less sensitivity to residual misalignment. The flexible couplings tended to minimize harmful vibration and related maladies caused by less-than-perfect rotating equipment alignment.
Eventually, pressures, temperatures and speeds reached a point at which thermal dislocations inherent in the foot-supported equipment became unacceptable for many users. This led to the development of a new type of turbine support which is still in use today, and which has come to be known as "centerline support". In centerline support arrangements, the support elements are separate from the turbine pressure casings to minimize temperature increases and thereby minimize thermal expansion. Further, they usually attach to the turbine at a location as close in elevation as possible to the location of the rotor shaft centerline, hence the terminology "centerline support". In fact, however, few have ever actually attached directly at the centerline.
The centerline support systems in current use in the industry all have numerous shortcomings, the most important being that they are not as rugged as the older integral foot support types. Most contemporary centerline support systems employ some sort of pedestal at the rotor shaft extension end of the turbine and a thin, flat plate-type column, called a "flex plate" or "flex leg", at the other end of the turbine. The pedestal is intended to be the primary means of fixing the turbine in place and of taking most of the external loads imposed by the piping carrying steam to and exhausting steam from the turbine. The flex plate is intended to carry the turbine deadweight at its respective end of the turbine, along with a portion of the vertical and lateral steam pipe loads. It is also intended to "flex" by elastic bending across its relatively thin section so that turbine longitudinal thermal expansion is only minimally resisted and thereby turbine stress and strain is minimized.
Contemporary pedestals are of generally two types. In one type, the pedestal is narrow-based and is attached to the sides or underside of the turbine shaft extension bearing housing. A primary disadvantage of this configuration is that the external piping loads are imposed on the bearing housing e.g. the load path from piping to foundation must pass through the bearing housing. This tends to distort the housing and cause bearing misalignment from one end of the turbine to the other. In the second type, the pedestal is wide-based and is attached to the turbine pressure casing near the centerline, outboard of the shaft extension bearing housing. The wide-base attachment results in increased stress and strain in both the turbine and pedestal from the "spreading" effect of the lateral thermal expansion of the hot turbine pressure casing. In the case of either pedestal type, they both are rather weak in resistance to torsion about the pedestal vertical axis, caused by steam piping loads. Such pedestals generally are comprised of ribbed and gussetted panels that occupy most of the end-face area of the turbine beneath and at either side of the shaft extension bearing housing. This expanse inconveniently obstructs access to the low-central shaft seal leakoffs and pressure casing and bearing housing drains. It also tends to inhibit free convective air flow around the bearing housing, which tends to promote heating of the bearing housing and its lubricants and also of the pedestal itself, thus significantly compromising the effectiveness of its ability to control centerline height, and to minimize thermal expansion.
The flex plate is invariably attached to the bearing housing at the turbine end opposite to the shaft extension. Thus, piping loads are again imposed on that bearing housing, exacerbating the bearing housing distortions and misalignments. The flex plate also tends to obstruct the low center of the inlet end of the turbine, again obstructing shaft seal leakoffs and casing drains and again tending in its flat, width expanse, to promote heating of the bearing housing and lubricant, and permitting undesirable turbine and turbine shaft thermal rise.
There remains, therefore, an unfulfilled need for an improved turbine support structure which offers all of the advantages of prior art structures without being saddled with the problems and disadvantages associated therewith.
Accordingly, a primary objective of the present invention is the provision of a turbine support structure which is extremely rugged, yet capable of providing both enhanced resistance to thermal centerline rise and minimal impedance to natural turbine casing thermal expansions in all directions.
Companion objectives include the provision of enhanced turbine shaft extension location control for optimum alignment to the driven equipment, isolation of bearing housings from external piping loads, minimum obstruction of turbine leakoff and drain connection zones, and minimum obstruction of convective cooling air currents about the turbine and its associated supports and bearing housings.